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The birth of memory

by MATT KWONG

What happens when a memory is born?

Johns Hopkins University scientists believe they recorded that very moment in the brains of rats. Their finding gives new insight into how neural “place cells” build mental maps of the world around us.

It’s a development that could lead to a better understanding of how spatial memory forms, and why it deteriorates among people with Alzheimer’s or brain trauma, said James Knierim, a neuroscience professor with the research team.

It’s like in Google Maps. You put a little red pin that marks locations. We think that’s what these place cells are. They’re acting like a pin, a marker of a spot.

Knierim and his colleagues monitored the neural activity of rats on a circular track and found that “place cells” in their brains lit up as the rodents stopped, scanned their surroundings, and continued to navigate the course.

The researchers believe those new signals indicated an exact moment when a rat was encoding a familiar experience as it circled back.

“It’s like in Google Maps. You put a little red pin that marks locations. We think that’s what these place cells are. They’re acting like a pin, a marker of a spot,” Knierim said.

The same place cells in the rat’s brain fired again as it returned to the same location multiple times, suggesting memory activation.

The Johns Hopkins team believes this real-time glimpse into the construction of a cognitive map in rats’ brains shows how the hippocampus — the brain’s seat of learning and memory — encodes new information.

That’s key to understanding more about Alzheimer’s because that horseshoe-shaped region also governs spatial navigation and is one of the first to suffer damage during early onset of the disease.

“One of the first symptoms of Alzheimer’s is things like getting disoriented, getting lost, and it’s well known that the hippocampus and entorhinal cortex, this is where you start seeing neurological damage indicative of Alzheimer’s,” Knierim said.

The hippocampus, where the place cells reside, encodes experiences “from sensory inputs about things you see, hear, smell in any moment of your life,” all of which could get incorporated into a spatial map, Knierim said.

“[The hippocampus] is sort of like an index at the back of a book. Maybe the memories aren’t stored there, but it tells you where in the brain to go and retrieve the memories to put it all together,” he explained.

The researchers believe the brain uses this internal spatial map formed by the hippocampus as “the organizing framework” for retrieving memories.

Without the ability to lay down pathways in a cognitive map due to brain damage, however, “you can’t re-experience an event in memory because you lost that spatial index that ties it all together,” Knierim said.

While Knierim said researchers had observed “spatial firing fields” in the neural activity of rats in the past, they had never before linked those new electrical signals with a specific behavior.

The scientists found that just as a rat stopped for several seconds to look around and notice a new spatial cue on a wall, the place cells began popping up in its brain and reacting to the change in environment.

“It saw something that caught its eye, and then there was this head-scanning behavior where the animal stops and pauses and looks at the world out there. That was the advance,” Knierim explained.

“This isn’t just a random noisy system where these fields come and go. They’re actually tied to a specific individual event — a correlate of the kinds of changes that occur in the human brain when we have an experience and lay it down as a memory.”

Knierim says that gaining insight into the birth of memories may help researchers test new therapies or treatments for Alzheimer’s disease.

“We’re not going to cure Alzheimer’s with the work we do, but we can understand from this what it is at this point that the brain is doing. That might help us understand why patients get these memory deficits when this brain system starts to break down due to the damage of Alzheimer’s,” he said.

The Johns Hopkins University team’s paper is published in the latest edition of the journal Nature Neuroscience.